Rollup merge of rust-lang#135275 - coolreader18:pin-as_deref_mut-relnotes, r=BoxyUwU

Add Pin::as_deref_mut to 1.84 relnotes

Resolves rust-lang#131243 - I think this got missed in the relnotes sweep or something.

``@rustbot`` label relnotes

Author: matthiaskrgr

RELEASES.md

@@ -60,6 +60,7 @@ Stabilized APIs
 - [`core::ptr::without_provenance_mut`](https://doc.rust-lang.org/stable/core/ptr/fn.without_provenance_mut.html)
 - [`core::ptr::dangling`](https://doc.rust-lang.org/stable/core/ptr/fn.dangling.html)
 - [`core::ptr::dangling_mut`](https://doc.rust-lang.org/stable/core/ptr/fn.dangling_mut.html)
+- [`Pin::as_deref_mut`](https://doc.rust-lang.org/stable/core/pin/struct.Pin.html#method.as_deref_mut)
 
 These APIs are now stable in const contexts